Many semiconductor fabrication processes are performed in process chambers such as plasma etch chambers, plasma deposition chambers, thermal processing chambers, chemical vapor deposition chambers, atomic layer deposition chambers, etc. These process chambers commonly use ceramic substrate heaters that support a substrate (e.g., a wafer) and provide heating of the substrate. In general, ceramic substrate heater materials provide good properties such as low thermal expansion, high temperature tolerance, a low dielectric constant, rigidity, and dimensional stability that make them preferred materials for many semiconductor applications. Ceramic substrate heaters are commonly composed of powdered metal oxides or nitrides combined with glass or frit particles. The mix of these materials is varied to generate a range of physical properties. The mixture is shaped into its desired form either by tape casting, powder pressing, roll compacting, or extrusion, and then sintered to form a hard crystalline structure.
Common ceramic materials for use in ceramic substrate heaters include alumina (Al2O3), aluminum nitride (AlN), silicon carbide (SiC), and beryllium oxide (BeO). Alumina is the most widely used ceramic material due to good availability, relatively low cost and stable physical properties. It is easy to fabricate into a range of shapes while remaining strong at high temperatures and it is available in a variety of purity levels. Beryllium oxide has the highest thermal conductivity available and has excellent dielectric strength needed for some applications, but it is available only in small sizes and safety can be a concern when dealing with toxic beryllium oxide powder. Silicon carbide is also highly conductive and offers an alternate to aluminum nitride and beryllium oxide, but caution must be used when selecting silicon carbide materials as dielectric strength can vary as temperature increases.
Aluminum nitride has high thermal conductivity that makes it an excellent choice where fast response or high levels of temperature uniformity are required, but it is costly to fabricate due to a high temperature firing requirement and material cost. Aluminum nitride substrate heaters are chemically “clean” substrates that meet the tough clean room environment for the semiconductor, medical and other very stringent applications. Furthermore, aluminum nitride substrate heaters used in semiconductor processing, can feature rapid heat up, easy temperature control, and excellent plasma durability.
Processing of substrates in a process chamber of a processing system can result in formation of material coatings on system components exposed to the process environment. For example, a coating can be formed on areas of a ceramic substrate heater that are not covered by a substrate. The partial coating of the ceramic substrate heater can lead to variations in the (thermal) emittance of the heater surfaces and can cause temperature non-uniformity and thermal stressing in the ceramic substrate heater. The thermal stressing can in time result in un-repairable mechanical damage such as cracking of the ceramic heater material. In addition, contacting a substrate with a substrate heater or a material coating on a substrate heater can result in backside contamination of a substrate. For example, copper diffusion in silicon devices is a well-known backside contamination problem, but other metals, for example ruthenium (Ru), can also be fast diffusers in silicon under moderate temperatures and bias conditions. The present inventors have recognized that improved methods are needed for reducing or preventing the above-mentioned problems associated with formation of material coatings on ceramic substrate heaters during substrate processing, while ensuring compliance with the strict requirements of processing semiconductor substrates.